Circuits for producing saw tooth currents



Dec. 9, 1952 c. L., FAUDELL 2,621,309

CIRCUITS FOR PRODUCING SAW TOOTH CURRENTS Filed April 6, 194 sSheets$heet 1 131 ,r E6 v 7 3/ 3 3o /nven/Z7r CHARLES; LESLIE FAUDELLDec. 9, 1952 c. L. FAUDELL 2,621,309

CIRCUITS FOR PRODUCING SAW TOOTH CURRENTS Filed April 6, 1949 3SheetsSheet 2 L? F/G.5.

/nyen for CHARLES LESLIE FAUDELL Affornev Dec. 9, 1952 c. L. FAUDELL2,621,309

CIRCUITS FOR PRODUCING SAW TOOTH CURRENTS Filed April 6, 1949 sSheets-Sheet s "HIP/I777! 7 CHARLES LESLIE FAUDELL Patented Dec. 9, 1952GI-RCUPIS FOR PRODUCING SAW TOOTH CURRENTS Charles LeslieFaudell, nearStoke Poges, England,

assigiior to Electr c '& Musical Industries Limited, Hayes, Middles'ex,England, a company of Great Britain Application April 6, 1949, SerialNo. 85 740 In Great Britain April 9, 1948 17 Claims. 1

This invention relates to circuits for producing a sawtooth current inan inductance of the type wherein means are provided for rectifying thevoltage pulses produced across said inductance due to the change ofcurrent during the short flanks of the sawtooth current waveform. Suchcircuits may be employed for example for deflecting the beam of acathode ra tube, for example in televisiontransmitting or receivingapparatus, the above-mentioned inductance being constituted by scanningcoils of the tube, and in such cases the relatively high steady voltagewhich is required to be applied to the final anode of the tube may bederived at least in part from the voltage pulses rectified asabove-mentioned.

The object of the invention is to provide an improved circuit of thistype.

According to the invention there is provided a circuit for producing asawtooth current in an inductance, wherein means are provided forrectifying the voltage pulses developed across said inductance duringthe short flanks of the sawtooth current waveform to provide a source ofvoltage, and comprising means for controlling the rate of change ofcurrent in said inductance during said short flanks whereby to controlsaid voltage.

In one form of the invention means are provided for adjusting theresonant frequency of the circuit in which said inductance is included.Said means may for example comprise a variable capacitor connected inshunt with said inductance or in shunt with a winding of a couplingtransformer via which the sawtooth current is fed to said inductance. Byadjustmentof said variable capacitor, the rate of change of the currentin said inductance during the short flanks of the sawtooth is varied,and since the amplitude of the voltage pulses is a function of said timethe steady voltage derived from said pulses is also varied. When forexample said steady voltage is applied to the final anode of a cathoderay tube the final anode voltage of said tube can in this way beadjusted without thereby substantially affecting the amplitude of thesawtooth deflecting current.

In another form of the invention means are provided whereby the resonantfrequency of the circuit in which said inductance is included is variedautomatically as the amplitude of the sawtooth current is varied,whereby to maintain the amplitude of said voltage pulses substantiallyconstant. Where the circuit is employed to provide deflecting currentand a finaI anode voltage for a cathode ray tube the amplitude of thesaw- 2 tooth current can in this way b adjusted, so as to vary themagnitude of the beam deflections without thereby substantiallyaffecting the final anode voltage.

In another form of the invention means may be provided for damping thecircuit including said inductance to a degree dependent on the ampli*tude of the sawtooth current, whereby the rate of change of current insaid coils during the short flank of the sawtooth current is varied sothat the amplitude of "said voltage pulses is substantially unafiectedby adjustment of the ampli-- tude of said saw-tooth current.

According to the invention also there is provided cathode ray tubeapparatus comprising a cathode ray tube provided with a permanent magnetelectron lens which assists in focussing the beam, means for developinga sawtooth current in the scanning coils of said tube, means forrectifying the voltage pulses produced in said coils during the fly-backstrokes, means for ap plying the rectified voltage to the final anode ofthe tube to assist in focussing the beam, and means for varying saidrectified voltage to adjust focussing of the beam without therebysubstan tially afiecting the amplitude of said sawtooth current.

In order that the said invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawings, which, in Figures 1 to 9,illustrate various circuit arrange ments according to the invention forproviding beam deflecting current and final anode voltage for a cathoderay tube.

In the various figures parts having the same functions have been giventhe same reference numerals.

Referring to Figure l, the circuit shown in cludes an electron dischargetube I, the anode 2 of which isconnected via the primary winding 3 of atransformer 4 to the positive terminal 5 of an H. T. voltage source (notshown). The secondary winding 6' of transformer 4 is connected to thescanning coils l of a cathode ray tube 30 The cathode 9 of tube l isconnected via cathode resistor in to an adjustable tapping on a resistorI I one end of which is connected to ground. A grid resistor I2 isconnected between said tap ping and the control electrode l3 of tube I.A diode rectifier M has its anode connected to anode 2 of tube l and itscathode connected to one electrode of a capacitor I5, the otherelectrode of which is connected to ground. A capacitor 16 is connectedbetween the high potential side of coils and the tapping on resistor II.In shunt with secondary Winding Ii, i.e. with the scanning coils l isprovided variable capacitor H. The control electrode I3 of tube I isconnected via a variable resistor IS and a capacitor I 9 to the anode ofan electron discharge tube 20, The anode of tube 28 is connected topositive H. T. terminal via a resistor 26 and the cathode is connectedto ground. A capacitor 2! is connected between the anode of tube 29 andground.

In operation, tube is normally non-conducting but is periodicallyrendered conducting by positive voltage pulses applied to its controlelectrode 2?. Capacitor 2| is charged via resistor 26 in the intervalsbetween said pulses, and upon the occurrence of each pulse capacitor 2|is discharged via tube 25, whereby a voltage of sawtooth waveform isdeveloped across capacitor 2I and is applied via capacitor I 9 andresistor I8 to the control electrode I3 of tube I. This results in avoltage pulse being periodically developed across the primary winding oftransformer 4, whereby the current in the scanning coils 7 increasesrelatively slowly in a substantially linear manner. Capacitors I6 and I!are inefiective during this increase of current. Upon the occurrence ofeach short flank of the sawtooth voltage, i. e. upon the cessation ofeach voltage pulse across the primary winding of transformer 4 thecurrent in the scanning coils I reverses relatively rapidly. The currentin said coils thus has a sawtooth waveform the long flanks of whichcorrespond to forward strokes of the electron beam and the short flanksof which correspond to fly-back strokes. During the rapid fall andreversal of the current in the scanning coils, there is generated acrossthe scanning coils I a voltage pulse the amplitude of which depends onthe amplitude of the deflecting current and on the rate of change of thecurrent during the fly-back stroke. Since the amplitude of this voltagepulse depends on the rate of change of current, that is to say on thetime during which the current falls and reverses, the amplitude of saidvoltage pulse likewise depends on the resonant frequency of the scanningcoil circuit and this frequency is determined partly by capacitor I6.The voltage pulses in primary winding 3 are rectified by diode I4, andthe rectified pulses are fed to capacitor I5, whereby a substantiallysteady voltage is produced across capacitor 65. Said steady voltage isapplied via conductor 23 to the final anode 29 of the cathode ray tube39, and serves in conjunction with a permanent magnet lens 3| to focusthe electron beam.

In order to vary the amplitude of the defiecting current in coils I thetapping on resistor II is adjusted, whereby the gain of tube I isaltered. If the resonant frequency of the scanning coil circuit remainsunaltered, variation in the amplitude of the deflecting current willresult in variation in the amplitude of the voltage pulses occurringduring fly-back, and therefore of the voltage pulses rectified by diodeI4. This means that the steady voltage across capacitor I5 will alsovary, thereby disturbing the focussing of the beam of the cathode raytube 38. However, since capacitor I6 is connected between coils I andthe tapping point on resistor II, as said tapping point is adjusted theeffective value of resistor I I in series with capacitor I6 is varied,whereby the effect of capacitor I6 as regards the resonant frequency ofthe scanning coil circuit is automatically varied in such sense that theamplitude of the rectified voltage pulses, and the magnitude of thevoltage across capacitor I5, is substantially unaltered by the change inthe amplitude of the deflecting current. The focussing of the electronbeam is thereby substantially unaifected by variation of the amplitudeof the deflecting current fed to the scanning coils. The variablecapacitor I! in shunt with the scanning coils provides a means forvarying the resonant frequency of the scanning coil circuit and hencethe rate of change of current in the scanning coils during flybackindependently of the means for varying the amplitude of the deflectingcurrent, so that the steady voltage may be independently varied. Asimple means of varying the focussing of the beam of cathode ray tube 36is thereby provided. Capacitor Il may if preferred be connected in shuntwith the primary winding 3 of transformer 4, as indicated in dottedlines. An alternative means for varying the focussing of the beam isafforded by variable resistor I8. This variable resistor, in conjunctionwith the capacity of the input electrodes of tube I, provides a networkof variable time constant, and adjustment of resistor I8 produces avariation in the final anode voltage by varying the rate of change,during fly-back, of the sawtooth voltage applied to tube I, therebyvarying the rate of cut-off of the tube I and hence the rate of changeof current in the coils 1 during flyback. Resistor I8 is ineflectiveduring the long flanks of the sawtooth current. Although capacitor I1and resistor I3 are both shown in Figure 1, they would not besimultaneously required in practice.

Figure 2 shows a modification of the circuit of Figure 1 in whichcapacitor I6 is omitted, and a variable capacitor 32 having a suitablelaw is connected in shunt with the scanning coils 1, capacitor 32 beingmechanically connected with the knob or like control member by which thetapping on resistor II is adjusted. During the long flanks of thesawtooth current capacitor 32 is ineffective, but during the shortflanks it becomes effective to control the resonant frequency of thecoil circuit. As the gain of tube I is varied to vary the amplitude ofthe deflecting current fed to the scanning coils the effective value ofcapacitor 32 is simultaneously varied so as to vary the resonantfrequency of the scanning coil circuit in such manner as to maintainsubstantially constant the amplitude of the voltage pulses set up duringfly-back. Focussing of the beam is effected by means of resistor I8 asabove described.

Figure 3 shows a further modification in which capacitor I6 is omittedand in which a capacitor 34 is connected in series with a variableresistor 35 between the anode and control electrode of tube I. Byadjustment of resistor 35 the degree of negative feedback at highfrequencies i. e. during the fiy-back is adjusted, thereby varying therate of change of the current in the scanning coils during the fly-backperiod. In this way, the final anode voltage may be adjustedindependently of the amplitude of the deflecting current. In this caseresistor I8 may be omitted. If desired resistor I8 may be retained andresistor 35 may be coupled with resistor II so that as resistor II isadjusted to vary the amplitude of the deflecting current, thesimultaneous variation of resistor 35 serves to alter the degree ofnegative feedback whereby to maintain the final anode voltagesubstantially unaltered, so that focussing is not disturbed. Resistorsl8 and 35 are ineffective during the long flanks of the sawtoothcurrent.

Referring now to Figure 4, which illustrates a further embodiment of theinvention, a cathode resistor l and grid resistor I2 are connecteddirectly to ground, resistor I0 having a by-pass capacitor 36. As in thecircuit of Figures 1 to 3 a voltage of sawtooth Waveform is arranged tobe applied to the control electrode [3 of tube I, so that current ofsawtooth waveform is produced in scanning coils l. The anode of tube 20is connected via a resistor 31 to a tapping on a potentiometer formed byresistors 38 and 39 connected in series between positive H. T. terminaland ground. In order to vary the amplitude of the sawtooth current fedto deflecting coils l, the amplitude of the sawtooth voltage applied tothe control electrode [3 of tube I is varied by adjusting the tapping onresistor 38. In order that the final anode voltage may not be affectedby such adjustment, a capacitor 40, which is ineffective during the longflanks of the sawtooth current, is connected between the tapping onresistor 38 and the junction of primary winding 3 with the anode ofdiode I4, so that as said tapping is adjusted the efiectiveness ofcapacitor 40 as regards determining the resonant frequency of thescanning coil circuit is varied automatically in such sense as tomaintain the amplitude of the voltage pulses across primary Winding 3during fly-back substantially unaltered upon a change in the amplitudeof the deflecting current. The final anode voltage is adjustableindependently by means of the variable resistor 18.

Figure 5 shows part of a circuit in which means are provided for dampingthe scanning coil circuit, the damping being controlled in accordancewith the amplitude of the deflecting current in the scanning coils. Inthis case capacitor I6 is made very much larger than would be requiredin an arrangement as shown in Figure 1, and there is provided in serieswith capacitor It a resistor 41 of such value that the combination ofcapacitor I6 and resistor 4| is such that there is little damping of thescanning coil circuit when variable resistor H is adjusted to providemaximum resistance. Upon a reduction of the effective value of resistorII to increase the amplitude of the deflecting current, increaseddamping is imposed on the scanning coil circuit, thereby preventing theampliture of the voltage pulses during fly-back from increasingsubstantially, whereby the final anode voltage, i. e. the steady voltageacross capacitor l5, remains substantially unaltered. The final anodevoltage can be independently adjusted by means of the variable resistorI8.

Figure 6 shows another circuit in which means are provided forcontrolling the rate of change of current in the scanning coils duringthe short flanks of the sawtooth voltage applied to the electrondischarge tube l which supplies the sawtooth current to the deflectingcoils. Normally said short flanks serve to reduce the current in saidtube rapidly to cut-ofl so that the deflecting current in the coils canreverse at a rapid rate unimpeded by current from said tube, which wouldtend to retard the reversal of the deflecting current. Said meanscomprises the variable resistor 42 which is connected between the anodeof tube and the junction of capacitors I9 and 2|, resistor 42 beingcoupled with the control member by which resistor ll is adjusted so thatas the effective value of resistor II is varied to vary the gain of tubel, the efiective value of resistor 42 is also varied, whereby to varysuitably the rate of discharge of capacitor 2| in such sense that theamplitude of the voltage pulses developed in the scanning coils duringthe short flank of the sawtooth current is substantially unaffected byadjustment of the amplitude of the sawtooth current. The resistor 42 issubstantially ineffective during the long flanks of the sawtoothcurrent.

In the arrangement of Figure 7, an inductive impedance 43 is insertedbetween resistor Ill and the variable resistor II. The value ofimpedance 43 is such that when the eflective value of resistor II iszero, there is substantial degeneration of the higher frequencycomponents, in the short flank of the sawtooth voltage generated by tube20. Resistor H has a shunt capacitative impedance 44 such that when theeifective value of resistor H is a maximum there is substantially nodegeneration of the short flank of the sawtooth voltage but maximum degeneration of the long flank. Thus by suitable choice of the Values ofimpedances '43 and 44 the rate of change of input voltage fed to tube 1during the short flank may be made to vary inversely with the amplitudeof the sawtooth voltage, thereby varying the degree of damp ing imposedby the internal impedance of tube I, which in turn will be proportionalto theamplitude of the deflecting current fed to the scanning coils.Impedance 43 has a shunt resistor 45 which serves to prevent unwantedresonances. Variable resistor l8 enables the final anode voltage to beindependently adjusted.

Referring to Figure 8, which illustrates a further embodiment of theinvention, a sawtooth voltage, which may be generated asabove-described, is applied via condenser l9, Variable re sistor l8, andfixed grid resistor 48, to the control electrode of electron dischargeamplifying tube l. The anode of tube l is connected to a tapping on theprimary winding of a transformer which is provided with two secondarywindings 5| and 52. The screen electrode of tube I is connected to thepositive H. T. ter-- minal 5 via a voltage dropping resistor 53' and toground via decoupling capacitor 54. The output of tube l comprises acurrent of sawtooth waveform, which is fed via transformer 59 to linescanning coils I, the ends of said coils being connected respectively toone end of secondary winding 5! and to a tapping on said secondarywinding.

One end of the primary winding of transformer 50 is connected to groundvia a capacitor 51 and the other end of said primary winding isconnected to the anode of diode [4 the cathode of which is connected viaa capacitor l5 to the anode of a diode 60, the cathode of which is alsoconnected to ground via capacitor 51. One end of secondary winding 5| isconnected to the H. T. terminal 5 and the other end of winding 5| isconnected to the anode of diode 65. The ends of secondary winding 52 areconnected to the terminals of the filament of diode M to supply heatingcurrent thereto and one end'of the winding 52 is connected via conductor28,

which includes a resistor 51, to the anode 29 of the cathode ray tube30.

The operation of the circuit is as follows: The positive potential forthe anode of tube l is derived from H. T. terminal 5 via the winding 5|of transformer 56, diode 60 and the primary winding of transformer 50,and is augmented by the voltage set up across capacitor 51 by means ofdiode 60 which rectifies the Voltage appearing across the whole of thesecondary winding 52 of transformer 50. The high voltage pulses whichare set up across the ends of the primary winding of transformer 59during the short flanks of the sawtooth current fed to coils I are fedto diode It and are thereby rectified and applied to one electrode ofcondenser I5. The other electrode of condenser I being connected tosecondary winding 5|, diode I4 also rectifies the pulses, of smallermagnitude, which appear across secondary winding 5|, and accordingly,the total voltage across capacitor I5 depends on the sum of the pulsesapplied to its two electrodes. This relatively high voltage, smoothed bycapacitor I5, is fed via resistor BI to the anode of the cathode raytube.

The diode 60 serves to damp the voltage pulse produced in the scanningcoil circuit at the end of the short flank of the sawtooth current andalso to improve the linearity of the long flank, since it conductsduring the whole or a great part of the duration of said long flank.

The resistor 6| serves to isolate the capacity of the cathode ray tubeelectrodes from the secondary winding 52 and in conjunction with thestray capacity of the cathode ray tube acts as a filter which serves toimprove smoothing of the line scanning pulses, and to reduce or preventradiation of line pulse harmonics from the conductor 28.

The amplitude of the sawtooth current fed to the scanning coils iscontrolled by negative feedback via a variable resistor II connected, inseries with fixed bias resistor I9, between the cathode of tube I andground. The control electrode of tube I is connected to ground via fixedresistor I2 and a capacitor 66, the ends of resistors I0 and I2 remotefrom the cathode and control electrode respectively being connectedtogether. Capacitor 66 is chosen so that there is no appreciabledegeneration of the higher frequencies during the short flanks of thesawtooth input voltage fed to tube I, so that when the effective valueof resistor II is increased the amplitude of the sawtooth current intube I is reduced but the effective rate of change of current in coils Iduring the short flanks of the sawtooth current is increased. Thus thetendency for a reduction in the amplitude of the sawtooth current tolead to reduction in the anode voltage of the cathode ray tube is atleast partly compensated by the tendency for the anode voltage toincrease due to a more rapid rate of change of current during said shortflanks.

Variable resistor I8 serves as the focussing control for the electronbeam. Focussing of the beam is effected by variation of the anodevoltage of the cathode ray tube in conjunction with the constant valueof the field produced by a permanent magnet electron lens 3|. Adjustmentof resistor I8 produces a variation in said anode voltage by varying therate of change of voltage during the short flanks of the sawtoothvoltage applied to tube I variable resistor I8 in conjunction with thecapacity of the input electrodes of tube I providing a network ofvariable time constant which controls the rate of cut-off of the tube I.

The ratio of resistors I8 and I2 is important in that it controls thetendency for the picture width to vary with variation in the anodevoltage and control electrode of tube I.

8 of the cathode ray tube (due to variations in the velocity of thebeam). For example an increase in the value of resistor I 8 reduces therate of change of the sawtooth voltage during the short flanks thereofand therefore also reduces the anode voltage of the cathode ray tube.Under these circumstances the picture width tends to increase but thistendency is opposed by the reduction in the input voltage of tube I inaccordance with the ratio of resistors I8 and I2.

The circuit of Figure 9 shows part of an arrangement which is similar tothat of Figure 8, except that capacitor 65 is omitted, and instead acapacitor 6! is connected between the cathode Said capacitor 61 serves,in conjunction with variable resistor 48, to provide a circuit ofvariable time constant which determines the effective input timeconstant of tube I. By adjusting resistor 48, the final anode voltagecan be adjusted. Cathode resistors II! and II provide a negativefeedback path which determines the effective capacity of the inputcircuit, including capacitor 67, of tube I. By varying resistor I I, thegain of tube I is varied, so varying the amplitude of the sawtoothcurrent applied to the scanning coils. At the same time, however, thevariation in the effective capacity of the input circuit of tube Icauses the rate of variation in the voltage at the control electrode oftube I during the fly-back periods to be altered in such sense that theamplitude of the voltage pulses rectified by diode I4 remainssubstantially constant so that the focussing of the electron beam issubstantially unaffected by variations in the amplitude of the scanningcurrent. In some cases, if the input capacity of tube I is suflicient,capacitor 6'! may be omitted.

What I claim is:

l. A circuit arrangement comprising an inductance, means for setting upa sawtooth current in said inductance, a rectifier connected to rectifyvoltage pulses set up across said inductance during the short flanks ofsaid sawtooth current to produce a unidirectional voltage, an elementassociated with said inductance substantially ineffective during thelong flanks of said sawtooth current but effective during said shortflanks to control the rate of change of current in said inductance, saidelement being connected and proportioned to control the resonantfrequency of the circuit in which said inductance is included, and meansfor eifectively varying the value of said element to produce a desiredrate of change of current in said inductance during the short flanks,whereby a desired value of said unidirectional voltage is produced.

2. A circuit arrangement according to claim 1, said element including acondenser coupled to said inductance.

3. A circuit arrangement according to claim 1, said means for setting upsaid sawtooth current including an electron discharge tube with itsoutput circuit coupled to said inductance, and means for periodicallycutting off the flow of current in said tube, whereby said elementserves to control the rate of cut-off of said current.

4. A circuit arrangement according to claim 3. including a time constantnetwork in the input circuit of said tube, said network including saidelement in the form of a variable resistor.

5. A circuit arrangement according to claim 1, including an electrondischarge tube for feeding said sawtooth current to said inductance, anda negative feedback path for said tube, said element being connectednegative feedback.

'9 to control the degree of 6'. A circuit arrangement comprising aninductance, means for setting up a sawtooth current in said inductance,a rectifier connected to rectify voltage pulses set up across saidinductance during the short flanks of said sawtooth current to produce aunidirectional voltage, an element associated with said inductancesubstantially ineffective during the long flanks of said sawtoothcurrent but effective during said short flanks to control the rate ofchange of current in said inductance, means for effectively varying thevalue of said element to vary said unidirectional voltage, means forcontrolling the amplitude of said sawtooth current, and means responsiveto adjustment of said controlling means for simultaneously adjusting theefiective value of said element to maintain said unidirectional voltagesubstantially constant;

'7. A circuit arrangement according to claim 6, said element, beingconnected and proportioned tocontrol the resonant frequency of thecircuit in which said inductance is included.

8. A circuit arrangement according to claim '7, said element being inthe form of a condenser coupled to said inductance.

9. A circuit arrangement comprising an inductance, means for setting upa sawtooth current in said inductance, a rectifier connected to rectifyvoltage pulses set up across said inductance during the short flanks ofsaid sawtooth current to produce a unidirectional voltage, an elementassociated with said inductance substantially ineffective during thelong flanks of said sawtooth current but effective during said shortflanks to control the rate of change of current in said inductance,means for effectively varying the value of said element to produce adesired rate of change of current in said inductance during the shortflanks, whereby a desired value of said unidirectional voltage isproduced, means for controlling the amplitude of said sawtooth current,means responsive to adjustment of said controlling means forsimultaneously adjusting the effective, value of said element tomaintain said rate of change of current and said unidirectional voltagesubstantially constant, an electron discharge tube for feeding saidsawtooth currentto said inductance, a negative feedback path connectedto said tube to control said unidirectional voltage, said feedback p-athincluding said adjustable element.

10. A circuit arrangement comprising an inductance, means for setting upa sawtooth current in said inductance, a rectifier connected to rectifyvoltage pulses set up across said inductance during the short flanks ofsaid sawtooth current to produce a unidirectional voltage, an elementassociated with said inductance substantially ineffective during thelong flanks of said sawtooth current but effective during said shortflanks to control the rate of change of current in said inductance,means for eflectively varying the value of said element to produce adesired rate of change of current in said inductance during the shortflanks, whereby a desired value of said unidirectional voltage isproduced, means for controlling the amplitude of said sawtooth current,means responsive to adjustment of said controlling means forsimultaneously adjusting the effective value of said element to maintainsaid rate of change of current and said unidirectional voltagesubstantially constant, and means for imposing damping on the circuitcontaining iii) 10 said inductance, said damping means comprising saidelement.

11. A circuit arrangement comprising a cathode ray tube, an inductance,means for setting up a sawtooth current in said inductance, a rectiflerconnected to rectify voltage pulses set up across said inductanc duringthe short flanks of said sawtooth current to produce a unidirectionalvoltage, an element associated with said inductance substantiallyineffective during the long flanks of said sawtooth current buteffective during said short flanks to control the rate of change ofcurrent in said inductance, means for eifectively varying the value ofsaid element to produce a desired rate of change of current in saidinductance during the short flanks, whereby a desired value of saidunidirectional voltage is produced, means for controlling the amplitudeof said sawtooth current, means responsive to adjustment of saidcontrolling means for simultaneously adjusting the efieotive value ofsaid element to maintain said rate of change of current and saidunidirectional voltage substantially constant, an electron dischargetube for feeding said sawtooth current to said inductance, a negativefeedback path connected to said tubeto control said unidirectionalvoltage, said feedback path including said adjustable element, saidcathode ray tube being provided with a permanent magnet fccussing lensto assist in focussing the electron beam of said tube, said elementbeing connected to serve as an adjustable beam focussing means.

12. Cathode ray tube apparatus comprising a cathode ray tube, meansincluding an inductance for deflecting the beam of said cathode raytube, means for driving a sawtooth current through said inductance,means for rectifying voltage pulses set up across said inductance duringthe short flanks of said sawtooth current to provide a highunidirectional operating voltage for said tube, means for adjusting theamplitude of the long flanks of said sawtooth current, an elementassociated with said inductance substantially ineffective during thelong flanks of said sawtooth current but effective during said shortflanks to control the rate of change of current in said inductance, andmeans coupling said element and said means for varying the amplitude ofsaid long flanks to maintain said unidirectional voltage substantiallyconstant with change in amplitude of said long flanks.

l3. Cathode ray tube apparatus comprising means including an inductancefor deflecting the beam of said cathode ray tube, an electron dischargetube for driving a sawtooth current through said inductance, meansincluding a variable resistor for adjusting the gain of said tube tovary the amplitude of the long flanks of said sawtooth current, meansfor rectifying voltage pulses set up across said inductance during theshort flanks of said sawtooth current to provide a high unidirectionaloperating voltage for said tube, and a capacitor connected to saidinductance and in series with said variable resistor, said capacitorbeing substantially ineffective during the long flanks of said sawtoothcurrent but effective during said short flanks to control the rate ofchange of current in said inductance, the magnitude of said capacitorand said variable resistance being so related that said unidirectionalvoltage remains substantially constant with change in amplitude of saidlong flanks.

14. Cathode ray tube apparatus comprising a cathode ray tube, meansincluding an inductance pulses set up across said inductance during theshort flanks of said sawtooth current to provide a high unidirectionaloperating voltage for said tube, and a capacitor connected between theoutput electrode of said driver tube and the output electrode of saidsawtooth generator tube, said capacitor being substantially inefiectiveduring the long flanks of said sawtooth current but effective duringsaid short flanks to control the rate of change of current in saidinductance, the

magnitude of said capacitor being proportioned to maintain saidunidirectional voltage substantially constant with change in amplitudeof said long flanks.

15. Cathode ray tube apparatus comprising a cathode ray tube, meansincluding an inductance for deflecting the beam of said cathode raytube, an electron discharge tube for drivig a sawtooth current throughsaid inductance, means for rectifying voltage pulses set up across saidinductance during the short flanks of said sawtooth current to provide ahigh unidirectional operating voltage for said tube, an inductiveimpedance in the oathode circuit of said tube substantially ineifectiveduring the long flanks of said sawtooth current but eflective duringsaid short flanks to control the rate of change of current in saidinductance, and a variable resistor in series with said inductiveimpedance to adjust the amplitude of the long flanks of said sawtoothcurrent, said inductive impedance being proportioned in relation to saidvariable resistor to maintain said unidirectional voltage substantiallyconstant with change in amplitude of said long flanks.

16. Cathode ray tube apparatus comprising a cathode ray tube, meansincluding an inductance for deflecting the beam of said cathode raytube, a driver tube for driving a sawtooth current through saidinductance, means for rectifying the voltage pulses set up across saidinductance during the short flanks of said sawtooth current to provide ahigh unidirectional operating voltage for said cathode ray tube, avariable resistor in the cathode circuit of said driver tube foradjusting the amplitude of the long flanks of said sawtooth current, anda capacitor connected in shunt with said variable resistor andsubstantially inefiective during the long flanks of said sawtoothcurrent but effective during said short flanks to control the rate ofchange of current in said inductance, the magnitude of said capacitorbeing proportioned in relation to said variable resistor to maintainsaid unidirectional voltage substantially constant with change inamplitude of said long flanks.

17. Cathode ray tube apparatus comprising means including an inductancefor deflecting the beam of said cathode ray tube, a driver tube fordriving a sawtooth current through said inductance, means for rectifyingthe voltage pulses set up across said inductance during the short flanksof said sawtooth current to provide a high unidirectional operatingvoltage for said cathode ray tube, a variable resistor in the cathodecircuit of said driver tube for adjusting the amplitude of the longflanks of said sawtooth current, and a condenser between the controlelectrode of said driver tube and its cathode and substantiallyineffective during the long flanks of said sawtooth current buteiTective during said short flanks to control the rate of change ofcurrent in said inductance to maintain said unidirectional voltagesubstantially constant with change in amplitude of said long flanks.

CHARLES LESLIE FAUDEIL.

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